The present invention relates to a granular polytetrafluoroethylene (PTFE) powder having a low-electrostatically-charging property and a preparation process thereof.
PTFE, particularly PTFE prepared by suspension polymerization is, after once pulverized to an average particle size of not more than 100 xcexcm, granulated by various granulation methods such as a dry granulation method, a method of granulation in the state of emulsion, a granulation method in the state of aqueous dispersion and a method of granulation by wetting and if necessary, subjected to shaping into a granular powder having an average particle size of about 100 to 600 xcexcm. The granular powder after the shaping is used as a molding powder for powder molding.
However since PTFE has electrically insulating property, it is easily charged when dry, its powder flowability is lowered and in addition, it sticks to a transportation pipe and a molding die. Once PTFE powder sticks thereto, complicated steps are required when changing to other product, and it is impossible to change from PTFE to other product particularly when PTFE is transported through the pipe.
Therefore it is proposed that after granulation, a granular powder is treated with an aqueous solution of ion-dissociative substance (electrolyte) such as an inorganic acid or inorganic salt to electrically neutralize static electricity (JP-B-53-13230). However in that method, though the charged granular PTFE powder can be once neutralized electrically, there is no effect on prevention of electrostatic re-charging caused by a friction at transporting and molding the powder after drying since the electrolyte is used.
The present inventors have found that a granular PTFE powder granulated in the presence of a surfactant has unexpectedly low-electrostatically-charging property, and as a result of further investigations, have found that a polar group of the surfactant has a function to prevent electrostatic charging and exhibits an electrostatic charging-preventing ability at the time when the powder is substantially dry, concretely when it is transported and is molded after drying. Thus the present invention was completed.
An object of the present invention is to provide a low-electrostatically-charging granular PTFE powder which can maintain an electrostatic charging-preventing ability even after dried substantially, and a preparation process thereof.
The present invention relates to a process for preparing a low-electrostatically-charging granular PTFE powder by contacting a granular PTFE powder to a polar group-containing organic compound having an electrostatic charging-preventing ability after the powder is dried substantially, and then drying the granular PTFE powder with the polar group-containing organic compound being left contained in the powder.
In that preparation process, it is preferable that after the polar group-containing organic compound in the form of an aqueous solution is contacted to the granular PTFE powder, the granular powder is dried without washing.
The effect of the present invention can be exhibited particularly when the granular PTFE powder contains no filler or when even if the filler is contained, the filler is an electrically insulating filler.
As a polar group-containing organic compound which has an electrostatically charging-preventing ability after dried substantially, preferred is a surfactant, particularly an anionic surfactant or nonionic surfactant. In case where the surfactant is used in the form of an aqueous solution, it is preferable that a content thereof is 1 to 30,000 mg/liter, particularly 25 to 10,000 mg/liter.
Further the present invention relates to the granular PTFE powder containing a polar group-containing organic compound in an amount of 10 to 300 ppm and having an electrostatic charge of not more than 50 V, preferably not more than 10 V.